Earth Planets Space, Vol. 62 (No. 6), pp. 525-544, 2010
K. Nagashima1, I. Kondo2, I. Morishita3, and R. M. Jacklyn4
1Solar-Terrestrial Environment Laboratory, Nagoya University, Chikusaku, Nagoya 404-8601, Japan
2Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashinoha, Kashiwa 277-8582, Japan
3Dept. of Information Management, Asahi University, 1851 Hozumi, Mizuho, Gifu 501-0296, Japan
4Antarctic Division, Department of Science and Technology, Kingston, Tasmania, Australia
(Received January 7, 2009; Revised January 30, 2010; Accepted February 26, 2010; Online published August 6, 2010)
The solar diurnal variation of cosmic rays was considered to be fully explained by the diffusion-convection theory. Recently, however, it has been found that the geographic latitude distribution of the yearly averaged diurnal variations observed with the neutron monitors and the muon telescopes on the ground does not agree with that expected from the theory. The difference between the two distributions is observed almost every year, and it is especially remarkable in the solar activity minimum period in the positive polarity state of the solar polar magnetic field, when the diurnal variation reduces its magnitude and shifts its phase towards the morning side. It is shown that such a difference is produced by the seasonal variation of the sidereal heliotail-in and helionose-in anisotropies with respective right ascensions of 6 h and 18 h according to the following process. Generally, if any sidereal anisotropy from the right ascension α is subject to the seasonal variation with its maximum and minimum at the times when the Earth is closest to and farthest from the source of the anisotropy, respectively, located through its direction at the boundary of the HMS, it produces a yearly averaged residual flux from 0 h LT in solar geographic polar coordinates regardless of its direction (α). This residual flux from 0 h LT produces the difference mentioned above.
Key words: Cosmic rays (CR), solar and sidereal anisotropies of CR, solar modulation of CR, seasonal variation of CR sidereal anisotropy, neutron monitors, polarity of solar magnetic field, heliomagnetosphere (HMS), CR acceleration on HMS boundary, major axis of HMS.